This invention relates to a semiconductor integrated circuit with an input protection circuit.
In the prior art CMOS integrated circuit constituting a memory, for example, an input protection circuit is connected to a power source input terminal (Vcc pin). The input protection circuit has npn parasitic bipolar transistor 11 formed in substrate (p-type substrate) of the integrated circuit as shown in FIG. 1. Transistor 11 is formed of p-type of substrate 10 and n.sup.+ -type diffusion layer 12 and 13. Further, a field oxide film 14 is formed on the surface of substrate 10 near n.sup.+ -type diffusion layer 12 and 13.
The input protection circuit functions to clamps input voltage Vi supplied to power source input terminal 15 to preset voltage level Vcc, thus preventing a voltage higher than the withstanding breakdown voltage from being applied to an internal circuit such as a memory circuit in the integrated circuit. More specifically, capacitor 16 and resistor 17 are connected to power source input terminal 15 so that voltage Vcc at a constant level can be supplied to the internal circuit even when input voltage Vi changes according to the time constant defined by capacitor 16 and resistor 17 as shown in FIG. 2. Voltage Vcc is equal to the sum of breakdown voltage Vbd at the junction of n.sup.+ -type diffusion layer 12 and p-type substrate 10 of bipolar transistor 11 and forward bias voltage Vf between substrate 10 and n.sup.+ -type layer 13. That is, voltage Vcc is determined by the withstanding voltage at the junction in transistor 11 or the withstanding voltage of the clamp circuit. Further, it is necessary to set voltage Vcc lower than the withstanding voltage of the elements constituting the internal circuit.
However, the withstanding voltage of the element of the internal circuit is determined by patterns for manufacturing the integrated circuit, various process conditions and the like, and therefore it will generally fluctuate to a great extent. For this reason, the withstanding voltage of the clamp circuit may be set higher than that of the element of the internal circuit and in such a case the clamp circuit may not correctly function as an input protection circuit. In order to solve this problem, it is considered to set the withstanding voltage of the clamp circuit to a sufficiently low level. In this case, however, it is necessary to perform a special manufacturing step.
In the integrated circuit having the prior art input protection circuit, the withstanding voltage of a clamp circuit constituting the input protection circuit may happen to be higher than that of the internal circuit of the integrated circuit. In order to obviate such a possibility, a manufacturing step for setting the withstanding voltage of the clamp circuit sufficiently low must be performed. This makes the manufacturing process of the integrated circuit complicated, incurring a problem of high manufacturing cost.